LocationPrivacy: include missing dep for Debian-based build

For some reason, installing diffoscope breaks the Debian/stretch versions
of gradle and gradle android plugin, adding this package fixes it.

metadata/info.guardianproject.locationprivacy.yml

@@ -49,7 +49,7 @@ Builds:
     timeout: 2000
     subdir: app
     sudo: apt-get -qy install --no-install-recommends -t stretch-backports diffoscope
-        apktool enjarify unzip xxd
+        apktool enjarify libstringtemplate4-java unzip xxd
     gradle:
       - yes
     prebuild: echo 'android.lintOptions.checkReleaseBuilds false' >> build.gradle